Membranes have been proposed to separate CO2 from other components in effluent gas streams. The recovery of carbon dioxide from effluent gas streams is propelled by multiple factors including the industrial carbon dioxide market, enhanced oil recovery (EOR), and governmental and industrial efforts to reduce greenhouse gas emissions reduction.
Many methods exist to remove CO2 from other components in effluent gas streams. When the effluent gas streams contain a high amount of CO2, the stream may be cooled to provide a liquid CO2 product. When the effluent gas streams contain a low amount of CO2, various methods have been used to increase the CO2 content prior to cooling, such as membrane separation or adsorption. Often when multiple methods are used, integration of the two methods to obtain more efficient energy savings has been overlooked. For example, U.S. Pat. No. 4,639,257 discloses recovery of carbon dioxide from a gas mixture using membrane separation and distillation. However, each step is effectively performed in isolation, with temperature and pressure adjustments before each membrane and distillation process. As disclosed in the '257 patent, the gas temperature and pressure for membrane separation is approximately 300 K (26.85° C.) and approximately 28 bar, respectively, whereas that for distillation is approximately −3° C. (270.15 K) to −40° C. (233.15 K) and approximately 1 to 3 bar, respectively. The energy requirements for such a process make it inefficient.
According to the U.S. Department of Energy, no current technology removes at least 90% of the CO2 from flue gases of existing pulverized coal (PC) power plants with less than a 35% increase in the cost of electricity. [DOE 2007]. The need remains for an economical, integrated CO2 recovery process.